The invention is directed to a single mode optical waveguide fiber designed for long repeater spacing, high data rate telecommunication systems. In particular, the single mode waveguide combines excellent bend resistance, low attenuation, and large effective area, Aeff, features that are desired for undersea applications.
A waveguide having large effective area reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, all of which can cause degradation of signals in high power systems. In general, a mathematical description of these non-linear effects includes the ratio, P/Aeff, where P is optical power. For example, a non-linear optical effect can be described by an equation containing a term, exp [Pxc3x97Leff/Aeff], where Leff is effective length. Thus, an increase in Aeff produces a decrease in the non-linear contribution to the degradation of a light signal.
The requirement in the telecommunication industry for greater information capacity over long distances, without electronic signal regeneration, has led to a reevaluation of single mode fiber index profile design. The genera of these profile designs, which are called segmented core designs in this application, are disclosed in detail in U.S. Pat. No. 4,715,679, Bhagavatula.
The focus of this reevaluation has been to provide optical waveguides which:
reduce non-linear effects such as those noted above;
are optimized for the lower attenuation operating wavelength range around 1550 nm;
are compatible with optical amplifiers; and,
retain the desirable properties of optical waveguides such as high strength, fatigue resistance, and bend resistance.
The definition of high power and long distance is meaningful only in the context of a particular telecommunication system wherein a bit rate, a bit error rate, a multiplexing scheme, and perhaps optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impact upon the meaning of high power and long distance. However, for most purposes, high power is an optical power greater than about 10 mw. In some applications, signal power levels of 1 mW or less are still sensitive to non-linear effects, so that Aeff is still an important consideration in such lower power systems.
A long distance is one in which the distance between electronic regenerators can be in excess of 100 km. The regenerators are to be distinguished from repeaters which make use of optical amplifiers. Repeater spacing, especially in high data density systems, can be less than half the regenerator spacing.
To provide a suitable waveguide for multiplexed transmission, the total dispersion should be low, but not zero, and have a low slope over the window of operating wavelength. In systems in which the suppression of potential soliton formation is important, the total dispersion of the waveguide fiber should be negative, so that the linear dispersion cannot counteract the non-linear self phase modulation which occurs for high power signals.
A typical application for such a waveguide fiber is undersea systems that, in order to be economically feasible, must carry high information rates over long distances without regenerators and over an extended window of wavelengths. The present invention describes a novel profile that is singularly suited to for use in these stringent conditions. The desired properties of the waveguide fiber for such a system are set forth in detail below.
The following definitions are in accord with common usage in the art.
The radii of the segments of the core are defined in terms of the index of refraction. A particular segment has a first and a last refractive index point. The radius from the waveguide centerline to the location of this first refractive index point is the inner radius of the core region or segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point is the outer radius of the core segment.
The segment radius may be conveniently defined in a number of ways, as will be seen in the description of FIGS. 1 and 2 below. In the case of FIG. 2, from which Tables 1 and 2 are derived, the radii of the index profile segments are defined as follows, where the reference is to a chart of xcex94 % vs. waveguide radius:
the radius of the central core segment, r1, is measured from the axial centerline of the waveguide to the intersection of the extrapolated central index profile with the x axis, i.e., the xcex94 %=0 point;
the outer radius, r2, of the first annular segment is measured from the axial centerline of the waveguide to the intersection of the first annular segment profile with a vertical line drawn through the xcex94 % point which is half of the xcex94 % difference between the first and the second annular segment profile;
the outer radius, r3, of the second annular segment is measured from the axial centerline of the waveguide to the intersection of the second annular segment profile with a vertical line drawn through the xcex94 % point which is half of the xcex94 % difference between the second and third annular segment profile;
the outer radius of any additional annular segments is measured analogously to the outer radii of the first and second annular segments; and,
the radius of the final annular segment is measured from the waveguide centerline to the midpoint of the segment.
The width, w, of a segment is taken to be the distance between the inner and outer radius of the segment. It is understood that the outer radius of a segment corresponds to the inner radius of the next segment.
No particular significance is attached to a particular definition of index profile geometry. Of course, in carrying out a model calculation the definitions must be used consistently as is done herein.
The effective area is
Aeff=2xcfx80(∫E2 r dr)2/(∫E4 r dr), where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light. The effective area is wavelength dependent. The wavelength at which the effective area is calculated is the wavelength at or near the center of the operating window for which the waveguide fiber is designed. More than one Aeff may be assigned to a waveguide fiber which operates over a range of the order of hundreds of nanometers.
Effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The relative index, xcex94 %, is defined by the equation,
xcex94 %=100xc3x97(n12-n22)/2n12, where n1 is the maximum refractive index of the index profile segment 1, and n2 is a reference refractive index which is taken to be, in this application, the refractive index of the clad layer.
The term refractive index profile or simply index profile is the relation between xcex94 % or refractive index and radius over a selected portion of the core.
The term xcex1-profile refers to a refractive index profile expressed in terms of xcex94(b) %, where b is radius, which follows the equation, xcex94(b)%=xcex94(b0)(1xe2x88x92[¦b-b0¦/(b1xe2x88x92b0)]xcex1), where b0 is the radial point at which the index is a maximum and b1 is the point at which xcex94(b)% is zero and b is in the range bixe2x89xa6bxe2x89xa6bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding is typically due to dopant diffusion in regions of rapid refractive index change.
Total dispersion is defined as the algebraic sum of waveguide dispersion and material dispersion. Total dispersion is sometimes called chromatic dispersion in the art. The units of total dispersion are ps/nm-km.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. Standard test conditions include 100 turns of waveguide fiber around a 75 mm diameter mandrel and 1 turn of waveguide fiber around a 32 mm diameter mandrel. In each test condition the bend induced attenuation, usually in units of dB/(unit length), is measured. In the present application, the bend test used is 5 turns of the waveguide fiber around a 20 mm diameter mandrel, a more demanding test which is required for the more severe operating environment of the present waveguide fiber.
The novel single mode waveguide fiber of this application meets the high performance telecommunication system requirements set forth herein.
A first aspect of the invention is a single mode optical waveguide fiber having a segmented core of at least two segments, surrounded by a cladding glass layer. The waveguide fiber has an effective area greater than 60 xcexcm2, and preferably greater than 65 xcexcm2, over the wavelength range of about 1530 nm to 1570 nm, attenuation at 1550 nm less than 0.25 dB/km and preferably less than 0.22 dB/km, a zero dispersion wavelength in the range of about 1565 nm to 1600 nm, a dispersion slope which provides a dispersion at 1560 nm more negative than about xe2x88x920.5 ps/nm-km and a preferred 1560 dispersion about xe2x88x922 ps/nm-km. Typically the slope is in the range of about 0.10 to 0.14 ps/nm2-km. The total dispersion of the waveguide fiber is in the range of about xe2x88x927.2 to xe2x88x923.9 ps/nm-km at 1530 nm. The mode field diameter is in the range of about 7.9 to 9.75 xcexcm over the 1530 nm to 1570 nm wavelength range.
These properties are achieved while maintaining good bend resistance, i.e., an induced bend loss no greater than about 5 dB/m, for 5 turns about a 20 mm mandrel. Also, cut off wavelength of fiber in cabled form is held in the range of about 1285 nm to 1500 nm. An added benefit is a polarization mode dispersion less than about 0.076 ps/(km)12, and typically about 0.04 ps/(km)12.
The index profiles of the respective segments can be any of those defined above, including an xcex1-profile, a step index profile, or a trapezoidal profile. Unless special steps are inserted in the process, the refractive index profiles will be rounded at points where the refractive index changes sharply. The rounding is due to diffusion of the dopant materials used to change the base glass refractive index. Thus any of these index profiles may be rounded at particular points. For example, a step index profile, having a positive xcex94 % will typically have rounded upper and lower corners.
In one embodiment of the invention, the core segments all have a positive xcex94 %. In another embodiment, the core comprises three segments, the first being an xcex1-profile, the second a step profile and the third a rounded step profile. Examples of this embodiment are set forth in Table 1 below.
In another embodiment of the invention, the core region comprises three segments and the center has been compensated for dopant diffusion so that the refractive index on or near the waveguide fiber centerline is not reduced relative to the remainder of the center profile. An example of such centerline compensation is shown in FIG. 3 wherethe dopant is germanium. The diffusion compensated embodiment shows an average improvement in polarization mode dispersion of about a factor of 5 relative to a comparable uncompensated waveguide fiber profile. The polarization mode dispersion of the novel waveguide fiber is less than 0.08 ps/(km)xc2xd and typically less than about 0.04 ps/(km)xc2xd. In a three segment embodiment, numbering the segments starting with 1 at the waveguide center, the segmented core is described by the parameters:
xcex941 % in the range of about 0.75 to 1.25;
r1 in the range of about 1.5 to 4.0 xcexcm;
xcex942 % in the range of about 0.00 to 0.15%;
xcex943 % in the range of about 0.2 to 0.7;
mid point radius r3 in the range of about 4 to 8 xcexcm; and, width of the third segment in the range of about 0.5 to 3 xcexcm.
A preferred range is:
xcex941 % in the range of about 0.85 to 1.20;
r1 in the range of about 2.0 to 3.5 xcexcm;
xcex942 % in the range of about 0.00 to 0.08%;
xcex943 % in the range of about 0.3 to 0.7;
mid point radius r3 in the range of about 5 to 7.5 xcexcm; and, width of the third segment in the range of about 0.8 to 2.0 xcexcm.
A more preferred embodiment is:
A1 % in the range of about 0.95 to 1.15;
r1 in the range of about 2.5 to 3.0 xcexcm;
xcex942 % in the range of about 0.00 to 0.04%;
xcex943 % in the range of about 0.3 to 0.7;
mid point radius r3 in the range of about 5 to 7.5 xcexcm; and,
width of the third segment in the range of about 0.8 to 1.5 xcexcm.
In another embodiment the total dispersion at 1560 nm is more negative than about xe2x88x921 ps/nm-km.
In yet another embodiment, centerline diffusion is either uncompensated or partially compensated so that there is an indentation of refractive index on centerline having a minimum xcex94 % not less than about 0.20 of xcex941 %. The indentation is typically of the shape of an inverted cone, i.e., the apex of the cone points downward, and the radius at the widest part of the cone is no greater than about 0.4 xcexcm.